In prior art, a useful form of an oscillator circuit is a crystal oscillator circuit. Such a circuit comprises and oscillator element (e.g., quartz cyrstal), a pair of electrodes attached to the oscillator element, and an inverting amplifying element connected across the electrodes. Such a circuit has a resonant frequency f.sub.R at which it will oscillate, this frequency depending in a rather complicated manner upon the effective parameters (inductance, capacitance, resistance) of the various elements, including the crystal. A capacitor, connected between one of the electrodes and ground, can be added for the purpose of changing ("tuning") the resonant frequency of the circuit.
In order to vary ("pull") the resonant frequency of a crystal oscillator circuit while it is oscillating, a varactor (variable capacitor) is substituted for the capacitor. During circuit operation, the capacitance of the varactor is varied by varying an external (control) voltage V.sub.C applied to the varactor, whereby the resonant frequency of oscillation of the circuit is pulled ("shifted") by an amount .DELTA.f.sub.R that depends upon the magnitude of V.sub.C. However, in many practical applications, the range (interval) of resonant frequencies ("pull range") at which the circuit can oscillate is limited to a narrower frequency interval (f.sub.max -f.sub.min) than is desired. For example, in some practical applications, it is desired to synchronize the clock circuits of two remotely located pieces of equipment by means of a separate VCXO located at each of them, one or both of the VCXOs having unavoidable resonant frequency fluctuations caused by local ambient temperature fluctuations. In such cases, the resonant frequency pull range of one of the VCXOs may not be wide enough to accommodate the unavoidable frequency fluctuations of the other VCXO.
Therefore, it would be desirable to have VCXO circuitry that will increase the pull range.